translate-c: Allow translating packed C structs iff ABI-sized

src/clang.zig

@@ -148,6 +148,9 @@ pub const APSInt = opaque {
 pub const ASTContext = opaque {
     pub const getPointerType = ZigClangASTContext_getPointerType;
     extern fn ZigClangASTContext_getPointerType(*const ASTContext, T: QualType) QualType;
+
+    pub const getTargetInfo = ZigClangASTContext_getTargetInfo;
+    extern fn ZigClangASTContext_getTargetInfo(*const ASTContext) *const TargetInfo;
 };
 
 pub const ASTUnit = opaque {
@@ -194,6 +197,12 @@ pub const ASTRecordLayout = opaque {
 
     pub const getAlignment = ZigClangASTRecordLayout_getAlignment;
     extern fn ZigClangASTRecordLayout_getAlignment(*const ASTRecordLayout) i64;
+
+    pub const getSize = ZigClangASTRecordLayout_getSize;
+    extern fn ZigClangASTRecordLayout_getSize(*const ASTRecordLayout) i64;
+
+    pub const getDataSize = ZigClangASTRecordLayout_getSize;
+    extern fn ZigClangASTRecordLayout_getDataSize(*const ASTRecordLayout) i64;
 };
 
 pub const AttributedType = opaque {
@@ -895,6 +904,11 @@ pub const TagDecl = opaque {
     extern fn ZigClangTagDecl_isThisDeclarationADefinition(*const TagDecl) bool;
 };
 
+pub const TargetInfo = opaque {
+    pub const getMaxPointerWidth = ZigClangTargetInfo_getMaxPointerWidth;
+    extern fn ZigClangTargetInfo_getMaxPointerWidth(*const TargetInfo) u64;
+};
+
 pub const Type = opaque {
     pub const getTypeClass = ZigClangType_getTypeClass;
     extern fn ZigClangType_getTypeClass(*const Type) TypeClass;

src/translate_c.zig

@@ -1166,8 +1166,8 @@ fn transRecordDecl(c: *Context, scope: *Scope, record_decl: *const clang.RecordD
             });
         }
 
-        if (!c.zig_is_stage1 and is_packed) {
-            return failDecl(c, record_loc, bare_name, "cannot translate packed record union", .{});
+        if (!c.zig_is_stage1 and is_packed and !isExternPackedRecord(c, record_decl)) {
+            return failDecl(c, record_loc, bare_name, "cannot translate packed record union (unless ABI sized)", .{});
         }
 
         const record_payload = try c.arena.create(ast.Payload.Record);
@@ -1204,6 +1204,53 @@ fn transRecordDecl(c: *Context, scope: *Scope, record_decl: *const clang.RecordD
     }
 }
 
+/// Check if the packed record is abi-compatible
+///
+/// Currently, a packed struct is only extern if it is "ABI-sized" and has one field
+fn isExternPackedRecord(c: *const Context, decl: *const clang.RecordDecl) bool {
+    assert(decl.getPackedAttribute()); // should be packed
+    // According to @Vexu, a packed record is only ABI compatible if it has one field
+    // See here: https://github.com/ziglang/zig/pull/12735#pullrequestreview-1095639816
+    const field_count = fieldCounter: {
+        var count: usize = 0;
+        var iter = decl.field_begin();
+        const end = decl.field_end();
+        while (iter.neq(end)) : (iter = iter.next()) {
+            // No need to deref here, but do it anyways for consistency
+            _ = iter.deref();
+            count += 1;
+        }
+        break :fieldCounter count;
+    };
+    if (field_count != 1) return false;
+    // For the purposes of translate-c, we consider a implement something "ABI sized"
+    // if it is smaller than a pointer
+    const struct_layout = decl.getASTRecordLayout(c.clang_context);
+    const ptr_bits = c.clang_context.getTargetInfo().getMaxPointerWidth();
+    // double check we're in bits
+    assert(ptr_bits >= 16);
+    assert(ptr_bits % 8 == 0);
+    const ptr_size = ptr_bits / 8;
+    const struct_size = struct_layout.getSize();
+    {
+        // verify the result from getSize() matches the result
+        // returned from getDataSize().
+        //
+        // According to clang doxygen, getDataSize() returns record
+        // size "without tail padding"
+        //
+        // Since this is a packed struct, this should match the regular size
+        const struct_data_size = struct_layout.getDataSize();
+        if (struct_size != struct_data_size) {
+            std.debug.panic(
+                "Expected getSize() {} to match getDataSize() {} (packed struct should have no padding)",
+                .{ struct_size, struct_data_size },
+            );
+        }
+    }
+    return struct_size <= ptr_size;
+}
+
 fn transEnumDecl(c: *Context, scope: *Scope, enum_decl: *const clang.EnumDecl) Error!void {
     if (c.decl_table.get(@ptrToInt(enum_decl.getCanonicalDecl()))) |_|
         return; // Avoid processing this decl twice

src/zig_clang.cpp

@@ -25,6 +25,7 @@
 #include <clang/AST/Attr.h>
 #include <clang/AST/Expr.h>
 #include <clang/AST/RecordLayout.h>
+#include <clang/Basic/TargetInfo.h>
 
 #if __GNUC__ >= 8
 #pragma GCC diagnostic pop
@@ -1832,14 +1833,23 @@ const char* ZigClangSourceManager_getCharacterData(const ZigClangSourceManager *
     return reinterpret_cast<const clang::SourceManager *>(self)->getCharacterData(bitcast(SL));
 }
 
-ZigClangQualType ZigClangASTContext_getPointerType(const ZigClangASTContext* self, ZigClangQualType T) {
+ZigClangQualType ZigClangASTContext_getPointerType(const ZigClangASTContext *self, ZigClangQualType T) {
     return bitcast(reinterpret_cast<const clang::ASTContext *>(self)->getPointerType(bitcast(T)));
 }
+const ZigClangTargetInfo *ZigClangASTContext_getTargetInfo(const ZigClangASTContext* self) {
+    const clang::ASTContext *ctx = reinterpret_cast<const clang::ASTContext *>(self);
+    return reinterpret_cast<const ZigClangTargetInfo *>(&ctx->getTargetInfo());
+}
 
+// TODO: UNUSED??
 unsigned ZigClangASTContext_getTypeAlign(const ZigClangASTContext* self, ZigClangQualType T) {
     return reinterpret_cast<const clang::ASTContext *>(self)->getTypeAlign(bitcast(T));
 }
 
+uint64_t ZigClangTargetInfo_getMaxPointerWidth(const ZigClangTargetInfo *self) {
+    return reinterpret_cast<const clang::TargetInfo *>(self)->getMaxPointerWidth();
+}
+
 ZigClangASTContext *ZigClangASTUnit_getASTContext(ZigClangASTUnit *self) {
     clang::ASTContext *result = &reinterpret_cast<clang::ASTUnit *>(self)->getASTContext();
     return reinterpret_cast<ZigClangASTContext *>(result);
@@ -2910,6 +2920,16 @@ int64_t ZigClangASTRecordLayout_getAlignment(const struct ZigClangASTRecordLayou
     return casted_self->getAlignment().getQuantity();
 }
 
+int64_t ZigClangASTRecordLayout_getSize(const struct ZigClangASTRecordLayout *self) {
+    auto casted_self = reinterpret_cast<const clang::ASTRecordLayout *>(self);
+    return casted_self->getSize().getQuantity();
+}
+
+int64_t ZigClangASTRecordLayout_getDataSize(const struct ZigClangASTRecordLayout *self) {
+    auto casted_self = reinterpret_cast<const clang::ASTRecordLayout *>(self);
+    return casted_self->getDataSize().getQuantity();
+}
+
 bool ZigClangIntegerLiteral_EvaluateAsInt(const struct ZigClangIntegerLiteral *self, struct ZigClangExprEvalResult *result, const struct ZigClangASTContext *ctx) {
     auto casted_self = reinterpret_cast<const clang::IntegerLiteral *>(self);
     auto casted_ctx = reinterpret_cast<const clang::ASTContext *>(ctx);

src/zig_clang.h

@@ -164,6 +164,7 @@ struct ZigClangStringLiteral;
 struct ZigClangStringRef;
 struct ZigClangSwitchStmt;
 struct ZigClangTagDecl;
+struct ZigClangTargetInfo;
 struct ZigClangType;
 struct ZigClangTypedefNameDecl;
 struct ZigClangTypedefType;
@@ -1049,6 +1050,7 @@ ZIG_EXTERN_C const char* ZigClangSourceManager_getCharacterData(const struct Zig
         struct ZigClangSourceLocation SL);
 
 ZIG_EXTERN_C struct ZigClangQualType ZigClangASTContext_getPointerType(const struct ZigClangASTContext*, struct ZigClangQualType T);
+ZIG_EXTERN_C const struct ZigClangTargetInfo* ZigClangASTContext_getTargetInfo(const struct ZigClangASTContext*);
 
 ZIG_EXTERN_C struct ZigClangSourceLocation ZigClangLexer_getLocForEndOfToken(struct ZigClangSourceLocation,
         const ZigClangSourceManager *, const ZigClangASTUnit *);
@@ -1077,6 +1079,8 @@ ZIG_EXTERN_C const struct ZigClangEnumDecl *ZigClangEnumType_getDecl(const struc
 
 ZIG_EXTERN_C bool ZigClangTagDecl_isThisDeclarationADefinition(const struct ZigClangTagDecl *);
 
+ZIG_EXTERN_C uint64_t ZigClangTargetInfo_getMaxPointerWidth(const struct ZigClangTargetInfo *);
+
 ZIG_EXTERN_C const struct ZigClangTagDecl *ZigClangRecordDecl_getCanonicalDecl(const struct ZigClangRecordDecl *record_decl);
 ZIG_EXTERN_C const struct ZigClangTagDecl *ZigClangEnumDecl_getCanonicalDecl(const struct ZigClangEnumDecl *);
 ZIG_EXTERN_C const struct ZigClangFieldDecl *ZigClangFieldDecl_getCanonicalDecl(const ZigClangFieldDecl *);
@@ -1106,6 +1110,8 @@ ZIG_EXTERN_C const struct ZigClangASTRecordLayout *ZigClangRecordDecl_getASTReco
 
 ZIG_EXTERN_C uint64_t ZigClangASTRecordLayout_getFieldOffset(const struct ZigClangASTRecordLayout *, unsigned);
 ZIG_EXTERN_C int64_t ZigClangASTRecordLayout_getAlignment(const struct ZigClangASTRecordLayout *);
+ZIG_EXTERN_C int64_t ZigClangASTRecordLayout_getSize(const struct ZigClangASTRecordLayout *);
+ZIG_EXTERN_C int64_t ZigClangASTRecordLayout_getDataSize(const struct ZigClangASTRecordLayout *);
 
 ZIG_EXTERN_C struct ZigClangQualType ZigClangFunctionDecl_getType(const struct ZigClangFunctionDecl *);
 ZIG_EXTERN_C struct ZigClangSourceLocation ZigClangFunctionDecl_getLocation(const struct ZigClangFunctionDecl *);